Medical diagnostic ultrasound occupies an important role in cardiac, fetal, and breast imaging, among other applications. For example, ultrasound's real-time nature and tack of ionizing radiation can make it more attractive than other alternatives. Unfortunately, high levels of image clutter can present a significant problem for certain patients, and diffraction effects can limit spatial resolution (e.g., to no better than hundreds of microns). For example, dynamic focusing can be used to approach the predicted theoretical diffraction limit. Using generally-available approaches, such focusing can unacceptably increase cost or complexity of the imaging system.
Generally, the resolution limit for ultrasound is assumed to be no better than λz/D, where λ can represent the ultrasound acoustic wavelength, z can represent the range to a target to be imaged, and D can represent an aperture size corresponding to the ultrasonic transducer. Thus, at least two routes can be used to improve resolution. The first can be to increase the operating frequency and thereby reduce the wavelength, λ. Such wavelength reduction works well at shallow depths, but can be limited by frequency dependent attenuation as the depth of the region of interest is increased. As the operating frequency increases, the signal to noise ratio (SNR) can also decrease, until it falls too low to form useful images. In one approach, coded excitation can mitigate this effect, but a tradeoff between resolution and SNR still exists. In another approach, resolution can be increased by expanding the imaging aperture, at least up to the point where an f-number (e.g., z/D) approaches 0.5. While aperture growth can be broadly effective, a practical limit can exist since the aperture cannot be made unreasonably large.
The past decade has seen two-dimensional (2D) arrays progress from research curiosities to common clinical tools. The majority of 2D array applications have been in cardiology for rapid volume acquisition. Such 2D array systems can include many active elements, and can include integrated circuitry within the probe assembly.